Relation of Uplifts to Thrusts in Rocky Mountains<xref ref-type="fn" rid="ch19fn1"><sup>1</sup></xref>

Abstract

The Rockies concerned are those of Montana, Wyoming, Colorado, New Mexico, and the Colorado Plateau portions of Utah and Arizona, east of the Paleozoic miogeosyncline. Evidence is presented that suggests that all the Rocky Mountain features of this region are the result primarily of Laramide vertical uplifts of oval or irregularly broad shape. They generally lack linear, narrow, or sinuous aspect. Some are conspicuously asymmetrical; others are fairly symmetrical. The structural relief ranges from 500 feet (Bowdoin dome) to 40,000 feet (Wind River uplift). Later Tertiary faulting has modified these Laramide uplifts considerably in places, and sediments and volcanic fields have partly covered some of them. When the thrust faults of the province are charted, they prove to be, for the most part, marginal to the uplifts. The uplifts of low and intermediate amplitude generally do not have associated border thrusts, but those in which Precambrian rock is exposed in the core commonly are bordered on one side or both by outwardly displaced thrusts. A firmer tie of uplift to border thrust is found in those where a structural relief of 20,000 feet or more exists. The above-noted relations suggest that vertical uplift was the primary deformation and that thrusting was a secondary lateral deformation caused by gravity sliding and flowing. Inasmuch as the basins were filled with sediments as the uplifts rose, it would appear that thrusting is likely not to be related directly to the structural relief of uplift over adjacent basins, but either to absolute relief at any one time as uplift exceeded sedimentation, or to density differences between basin sediments and the rocks of the uplift. Anticlines suitable for oil and gas accumulation seem to be related to the marginal gravity creep from the uplifts. The locale is one of interplay of thrusting and folding of the surficial strata, and of sedimentation. The Rocky Mountain region of uplifts is essentially the igneous province of alkalic and calcalkalic rocks. Consideration of the origin of these rocks, of the nature of the uplifts, and of geophysical data bearing on deep-seated crustal structure, leads the writer to postulate that the uplifts are due to megasills or megalaccoliths deep in the silicic (granitic) layer, perhaps near the boundary of the silicic and basaltic layers. It is expected that model experiments will indicate size, shape, and depth of intrusion necessary to produce the various surface structures, and the nature of the border faults.